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Dep. of Crop Science, Univ. of Guelph, Guelph, Ontario, Canada N1G 2W1
* Corresponding author (E-mail: hearl{at}crop.uoguelph.ca).
Recent evidence suggests that chlorophyll fluorescence techniques can be used to determine photosynthetic rates of maize (Zea mays L.) leaves much more rapidly than is possible with conventional gas exchange methods. However, the accuracy of such measurements depends directly on the accuracy with which leaf absorptance of incident photons in the PAR (photosynthetically active radiation) region can be estimated. Our objectives were (i) to monitor changes leaf ahsorptance in a typical maize crop over the growing season, and (ii) to determine if absorptance of maize leaves could be accurately estimated with a SPAD 502 hand-held chlorophyll meter (Minolta Corporation, Ramsey, NJ). Leaves from six leaf positions were harvested on eleven dates between 55 and 134 d after planting. An integrating sphere and field-portable spectroradio meter were used to measure reflectance and transmittance of leaf tissue samples at 5-nm intervals between 400 and 700 nm, and a SPAD value was also determined for each sample. Greening of young leaves was associated with decreases in both reflectance and transmittance in the middle wavelengths of the PAR region, while leaf senescence was associated with increases in both reflectance and transmittance, primarily in the longer wavelengths of the PAR region. Leaf absorptance of incident photons in the PAR region by healthy, fully expanded leaves ranged from approximately 0.88 to 0.91; much lower values were observed for young, chlorotic, or senescing leaves. Regression analysis revealed a strong relationship (r2 = 0.98) between leaf absorptance and SPAD value, suggesting that the SPAD meter could be used to provide a rapid estimate of leaf absorptance in the field.
Received for publication January 26, 1996.
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